1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic tire having an excellent steering stability at wet condition and suitable for competitive sports, particularly racing cart.
2. Description of Related Art
In wet-type tires for competitive sports such as racing cart and the like is generally adopted a block-type tread pattern placing importance on a gripping stability when a water volume on a road surface is large in view of the safety. In such a tread pattern, however, there is a problem that as the water volume is decreased by the change of weather condition, the lateral slippage at the corner becomes large due to the lacking of rigidity in the blocks of the pattern and the performance deterioration is caused.
Since a racing cart is light in the load, it frequently adopts a fine block pattern mainly comprising four main grooves as a front tire or five main grooves as a rear tire continuing in a circumferential direction of the tire. As a result, as the water volume might become less, there is caused a remarkable performance deterioration that the lateral slippage becomes larger due to the phenomenon that the block rigidity is lacking or the lowering of lateral force produced in the cornering accompanied with the progress of wearing at shoulder blocks.
On the other hand, as a largish block pattern is adopted by placing importance on the performance when the water volume is small, the total groove area in the limited ground contact area is decreased, and hence the hydroplaning phenomenon is liable to be easily caused under a condition that the water volume is large to finally bring about the deterioration of the steering stability. In the tires for wet condition of changeable water volume on the road surface, therefore, it is a significant matter how to stably develop the gripping performance even in a broad range of water volume conditions.
As a technique on the improvement of the tread pattern in the pneumatic tire, for example, JP-A-5-338415 discloses a pneumatic tire wherein circumferential grooves extending in the circumferential direction of the tread and widthwise grooves extending obliquely upward are arranged in the tread and the widthwise groove is comprised of first groove segment and second groove segment to improve the resistance to uneven wear while ensuring the excellent gripping performance and high gripping force on the road surface in the high-speed cornering. Also, JP-A-9-226322 discloses a pneumatic tire having a tread pattern provided with two kinds of directional slanting grooves having given low inclination angle and high inclination angle with respect to the circumferential direction. Furthermore, JP-A-4-43105 and JP-A-4-126612 disclose a pneumatic tire wherein circumferential straight grooves extending in the circumferential direction and a plurality of slant grooves extending obliquely upward from a central region of the tread toward an end of the tread and opening to the tread end are arranged at given positions, respectively to improve the steering stability on a dry road surface and the drainage property on a wet road surface. In addition, JP-A-4-50006 discloses a pneumatic tire wherein curved grooves of hyperbolic shape are arranged at given angles on both sides of a main groove disposed in the circumferential direction as a technique of improving the tread pattern for the purpose of improving the drainage property. Moreover, JP-A-2002-103920 discloses a technique wherein position-adjusted and extended chamfer portions are arranged under given conditions in order to realize a tread of a pneumatic tire suitable for wide applications on paved road and off-the-road.
As to the improvement of the tread pattern in the pneumatic tire used under wet conditions, there are made various examinations as mentioned above, but it is demanded to develop pneumatic tires more improving the steering stability from the requirement of further improving the performances.
In the conventional wet tires for the racing cart, since the block pattern comprising four main circumferential grooves as a front tire or five main circumferential grooves as a rear tire is commonly used as previously mentioned, there is a problem that as the water volume becomes less, the wearing of the blocks located at the inside of the mounting becomes fast to bring about the lowering of the lateral force produced and the lowering of the braking force. As a counter-measure, it is mentioned to make the block large, which decreases the total groove area in the limited ground contact area of the tire for the racing cart being light in the load as long as it is positioned on an extension of the conventional block pattern based on the main circumferential grooves, so that it has not yet led to prevent the problem of easily causing the hydroplaning phenomenon or the occurrence of uneven wear along the main circumferential groove.